The overall goal of this project is to determine how the mechanisms normally used for head stabilization, including signals from vestibular otoliths and canals and their interaction with passive mechanical properties, are modified by the simultaneous demands of a voluntary visual processing task. Specifically, this proposal explores how elderly and young adults coordinate their head and trunk movements when confronted with simultaneous visual acuity and postural stabilization tasks. We hypothesize that if vision controls head position, there should be no conflict - the head will always work to stabilize the visual field. But, if the head is part of a whole body synergy in response to ascending forces, a choice might have to be made. We would expect different neural and biomechanical characteristics of postural stabilizing actions when stabilization of the head is regulated with respect to space (i.e., visual orientation) versus with respect to trunk (i.e., body orientation). Responses during anterior- posterior and lateral translations, the planes of motion where significant power has been demonstrated during functional locomotion, will be examined in subjects seated with the trunk fixed or free to move and when standing. Subjects will be presented with a stereo visual field containing a stable focal image, an image that is in phase with the rotation of the trunk, and a visual image that moves out of phase with the trunk. Smoothed position ramps, sinusoids, and PRBS stimuli will be used and will permit us to derive both time and frequency domain characteristics of the response. Given that head4o-visual target and head-to-body stabilization can operate without problems together, we expect-to find, particularly in the elderly, that there are limits (e.g. mechanical) which call for some decision to favor one function at the cost of the other. Both the spatial organization and response magnitudes of the neck and trunk muscles will be analyzed to characterize the neural response strategies. Models that incorporate reflex control of head and neck position with descending controls and system mechanics will be used to assist in the analysis and prediction of postural control mechanisms. Clarification of relations between the mechanisms responsible for postural stabilization should assist clinicians in evaluating the causes of falling and balance disturbances, and should improve therapeutic interventions applied both to the elderly and to patients with central nervous system deficit.